an adiabatic process is one whereby there is no transfer of heat between the system and its surroundings, thus Q (heat) = 0
an isoentropic process is one whereby entropy remains constant. if a process is both adiabatic and reversible, then it is considered as isoentropic.
adiabatic expansion can occur in a well-insulated system. neglecting kinetic energy, electrical energy, etc, drop in enthalpy of the system is effectively converted to work. an adiabatic expanion is thus considered to be most expanion. (dH = Q + W, where Q = 0) even in an ideal engine, it's not 100% efficient. this is well-demonstrated in Carnot'e engine.